Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 210958

Area Forecast Discussion
National Weather Service Seattle WA
258 AM PDT Tue Aug 21 2018

.SYNOPSIS...Offshore wind flow will continue to bring smoke into
Western Washington through Wednesday. A turn to cooling onshore
flow is expected Wednesday night and Thursday. Smoke is expected
to gradually decrease on Thursday. Onshore flow will continue
Friday and Saturday. An upper low could move across the area on
Sunday, bringing a few showers that could linger into Monday.


.SHORT TERM...Low level offshore flow today will switch to
onshore during the day Wednesday. This should help clear smoke
out of the area Wednesday night and Thursday. Until then, smoke
and poor air quality will continue. An air quality alert is in
effect through Wednesday. It may need to be extended into
Thursday. That decision will be made on Wednesday.

Highs today will be above normal but 5-10 degrees cooler than
guidance due to all the smoke. The coast will cool to near normal
on Wednesday as onshore flow develops but the interior should see
minor cooling at best.

An upper trough will move over the area on Thursday but it looks
dry for the most part. A marine push will spread low clouds
inland Wednesday night and cool highs to near or slightly below
normal all areas on Thursday. There could also be some drizzle
Wednesday night and Thursday morning - mainly towards the coast.

.LONG TERM...An upper trough will push east of the area Friday
with low level onshore flow continuing and slightly below normal
highs. There might be a few showers but mostly it looks dry with
morning clouds and partial afternoon sunshine.

A couple of additional upper level troughs will swing through the
area this weekend and into early next week. This will maintain
cool weather across the area with a chance of showers. The best
chance of showers appears to be on Sunday although confidence in
the details is not high for now. Schneider


.AVIATION...Upper level ridging continues to build into the
region today with northeast flow aloft. Low level flow has turned
offshore overnight with little in the way of stratus near the
coast. Smoky conditions will continue today with a substantial
smoke layer above 4,000 feet. Surface visibilities in smoke are
expected to be generally in the 3- 5SM range.

KSEA...Visibility 3-5SM in FU with a smoke layer OVC030-050.
Surface winds N-NE winds to 10 kt. 27


.MARINE...A thermally induced trough of low pressure along the
WA coast early this morning will weaken during the day. The flow
will become nearly flat overnight. Strong onshore flow will
develop on Wednesday for the possibility of gale force westerlies
over parts of the Strait of Juan de Fuca Wednesday night. Onshore
or westerly flow will then continue on Thursday but will be a
little weaker.

Meanwhile, smoke from wildfires is expected to begin slowly
clearing from the west on Wednesday. It should be completely out
of the area by Friday.




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